Human T-cell lymphotropic virus type I (HTLV-I) is associated with two human diseases, adult T-cell leukemia (ATL) and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). HTLV-I Tax1 is released from infected lymphocytes and functions as an extracellular cytokine to stimulate gene expression in, and proliferation of, uninfected lymphocytes. The human parathyroid hormone related protein (PTHrP) gene has been analyzed in HTLV-I infected and uninfected cells to identify transcriptional regulatory sequences which may be a target for intracellular or extracellular Tax1. The analysis of PTHrP gene expression is important since humoral hypercalcemia of malignancy (HHM) is closely linked to PTHrP synthesis and secretion. Several types of human cancers, including ATL are frequently associated with hypercalcemia. PTHrP shares with the parathyroid hormone (PTH) the ability to interact with the PTH/PTHrP receptor and, consequently, to induce bone absorption and increased calcium reabsorption in the kidney, which eventually results in an increased calcium level in the blood. The PTHrP promoter is stimulated by several extracellular cytokines and growth factors. Expression of the parathyroid hormone related protein is regulated by two distinct promoters, P1 and P2. In HTLV-I transformed lymphocytes, RNA synthesis is initiated primarily at the P2 promoter. Using deletion and site-specific point mutations, we have identified a promoter proximal sequence (-72 to -40) which is important for Tax1 transactivation. This promoter proximal element contains binding sites for both Ets1 and Sp1. Mutation of the Ets1 GGAA core motif or the Sp1 CACCC motif significantly reduces transcriptional activity of the PTHrP promoter. These biochemical studies provide important insight into PTHrP gene regulation. This information is being utilized to develop strategies to theraputically regulate PTHrP expression and the development of hypercalcemia in cancer patients.